This invention relates to processes and apparatus for producing magnesium metal by electrolysis from magnesium chloride fed to an electrolytic cell for decomposition into the product metal and chlorine gas. In particular, the invention is directed to new and improved processes and apparatus of this type, wherein magnesium chloride is supplied to the cell as a feed comprising or including one or more magnesium chloride ammoniates.
Electrolytic cells for producing magnesium metal from MgCl.sub.2 are well known and widely employed in present-day commercial practice. Typically, in such a cell, the MgCl.sub.2 is dissolved in a molten salt electrolyte comprising a mixture of alkali metal and alkaline earth metal chlorides, in which some fluorides also may be present. Magnesium metal deposits in molten state on the cell cathode(s) and chlorine gas is generated at the anode(s) within a cell chamber; since both the metal and the gas are lighter than the electrolyte, both migrate upwardly. The magnesium metal is transported to a locality outside the cell chamber for collection and periodic removal, while the chlorine gas is separately collected and withdrawn above the cell chamber. Suitable means for controlling the cell temperature and/or electrolyte level may also be incorporated in the cell structure; one temperature control arrangement is described in U.S. Pat. No. 4,420,381.
It is conventional to provide a path for circulation of the electrolyte into the cell chamber, upwardly through the generally vertical space(s) between the facing anode and cathode surfaces (each such space being herein termed the anode-cathode distance, or "ACD"), out of the cell chamber to the metal-collecting locality, and back into the cell chamber again, provision also being made for feed of fresh quantities of MgCl.sub.2 to the electrolyte before its return to the latter chamber. Such circulation of electrolyte may be effected by a pump, but it is convenient to take advantage of the gas-lift effect of the plume(s) of generated chlorine bubbles rising from the anode(s) to provide the motive power for electrolyte circulation.
For this reason, and also for the sake of economy in electrical power consumption, it is desirable that the width of each ACD (i.e., the horizontal spacing between the facing active anode and cathode surfaces of each electrode pair) be as small as possible. At the same time, to achieve reasonably high current efficiency, recombination of the produced magnesium with the liberated chlorine gas in the cell must be minimized. Among the cells affording these desired features of arrangement and operation are the so-called multipolar cells described, for example, in U.S. Pat. Nos. 4,514,269, 4,518,475, 4,560,449, 4,604,177 and 4,613,414, the disclosures of which are incorporated herein by this reference. Multipolar cells as described in the cited patents are characterized by a multiplicity of closely spaced electrodes, with interelectrode spacings (ACD widths) typically between 4 and 25 mm, more usually 4 to 15 mm. The present invention will be described herein, for purposes of illustration, as employed with cells of this type, although in its broader aspects the invention is not limited thereto.
Magnesium cells are limited in productivity by the Joule effect of the DC current employed for electrolysis. Modern cells of multipolar design operate at 2-5 kWh/kg Mg Joule effect compared to 7-13 kWh/kg Mg for older designs at similar current densities. While higher current densities would be uneconomical for older cell designs, because the unit power consumption is too high, multipolar cells could be operated economically at higher current densities (with enhanced productivity) but for the fact that heat dissipation capability sets maximum production limits for a given cell size. For good current efficiency, cell temperature must be kept within a narrow range, e.g. within .+-.2.degree. C. of an optimum value, except during metal tapping; since the heat generated in a given cell increases with increasing current density, the cell temperature will rise beyond the optimum range if the current density exceeds the limit imposed by the ability of the cell to maintain thermal balance by dissipating the generated heat.
One type of cell uses a hydrated magnesium chloride feed and its heat balance is designed accordingly. However, the hydrated feed results in rapid graphite (anode) consumption and sludge generation. These two undesirable side effects become intolerable in the operation of modern multipolar cells owing to the very small ACD in such cells; as the anode is consumed, the voltage rises and the heat generation eventually exceeds that which can be controlled by present-day thermostatic means. The natural wear of the anodes and the bipolar electrodes is the main cause of cell shutdown, when the limit imposed by thermal balance controls is reached.
A high-purity feed of anhydrous MgCl.sub.2 is commonly used for multipolar cells as well as for other types of magnesium-producing electrolytic cells. Since magnesium chloride occurs in natural and artificial brines, and in ores such as carnallite and bischofite, in a polyhydrated form, e.g. as hexahydrate, it is necessary to remove the water of hydration in order to obtain the desired anhydrous feed. Ammoniated magnesium chloride compounds are usable as material for producing anhydrous magnesium chloride, and a variety of techniques have heretofore been used or proposed for treating magnesium chloride hydrates to obtain such ammoniated compounds (usually the hexammoniate, MgCl.sub.2.6NH.sub.3), as described, for example, in U.S. Pat. Nos. 2,381,994, 3,092,450, 3,352,634, 3,966,888, and 4,228,144. One such process, affording particular advantages, is described in copending U.S. patent application Ser. No. 08/043,184, filed by applicant herein jointly with J. V. Sang and R. J. R. Lemay on Apr. 6, 1993, and assigned to the same assignee as the present application. The ammoniation processes, however, involve a final step of calcining the magnesium chloride ammoniate or ammoniates to achieve anhydrous MgCl.sub.2 for feed to an electrolytic cell; the calcination is expensive because it requires the supply of large amounts of heat to release the ammonia in gaseous form.